I'm new to databases, but I think I finally have a situation where flat files won't work.
I'm writing a program to analyze the outcomes of multiplayer games, where each game could have any number of players grouped into any number of teams. I want to allow players can win, tie, or leave partway through the game (and win/lose based on team performance).
I also might want to store historical player ratings (unless it's faster to just recompute that from their game history), so I don't know if that means storing each player's rating alongside each game played, or having a separate table for each player, or what.
You didn't mention any need for locking mechanisms where multiple users may be competing to write the same data to the same resource (a database record or file in the case of flat files) simultaneously. What I would suggest is get a good book on database design and try to understand normalization rules in depth. Distributing data across separate tables have a performance impact, but they also have an effect on the ease-of-use of query construction. This is a very involving topic, and there's no simple answer to it. That's why companies hire database administrators to keep their data structures optimized.
You might want to look at SQLite, if you need a lightweight database engine.